In an email to Flightglobal, the Japanese Ministry of Defence outlined Tokyo's position on price increases. It said that if a price rises "without valid reasons, there is a possibility that a procurement could be cancelled".

"This message is conveyed to the US side occasionally. MoD will continue to request the US government to deliver the aircraft at the price in accordance with the content of the proposal by the period requested."

In December, Japan announced it had selected the F-35 as winner of its 42-aircraft F-X competition, the other competitors being the Boeing F/A-18E/F Super Hornet and Eurofighter Typhoon.

Lockheed has said the conventional take-off and landing (CTOL) F-35A variant can be delivered for an average unit cost of about $75 million, although that number assumes the USA and eight partner countries order more than 3,100 jets during the next 25 years.

Japan's concerns come at a time when procurement decisions in several countries could influence costs associated with the F-35. In mid-February, the US Air Force cut its planned fiscal year 2013 buy of F-35s to 19 aircraft from 24, although its total planned buy remained unchanged at 1,700.

Italian defence sources told Flightglobal in early February that Rome could obtain 20-30 fewer F-35s than the 131 originally planned. Italy's planned procurement of 69 CTOL F-35As and 62 short take-off and vertical landing F-35Bs has come under fire in the Italian parliament for its anticipated total cost of €17.4 billion ($23.4 billion), with €2 billion already spent on the acquisition.

Australia is also reviewing its planned purchase of 100 F-35s. While stressing it is keeping all options open, Canberra has indicated it could reduce its planned buy in favour of more Super Hornets. Industry observers suggest Canberra could obtain between 12 and 24 additional Super Hornets to join its 24 F/A-18Fs.

In addition, the F-35 is competing against the Boeing F-15 Silent Eagle and Eurofighter Typhoon in South Korea's F-X III competition for about 60 fighters. Given its stealth treatments and Seoul's familiarity with the F-15K, the F-15SE is in a better position to defeat the F-35 than was the case with the Super Hornet in Japan's F-X contest.

A 60-aircraft F-35 victory in South Korea could provide a substantial boost to the F-35's cost competitiveness.